(1) Field of the Invention
This invention relates to an inorganic filler derived from a clay mineral of the montmorillonite group and having an excellent whiteness and a high oil absorption, and a process for production thereof. Particularly, the present invention relates to a filler for lightweight paper which well prevents strike-through of paper and has excellent opacity and acid resistance, and a process for production thereof.
(2) Description of the Prior Art
Decreasing of the weight of paper such as newsprint has been rapidly attempted to save wood resources, transportation costs, etc. Lighter weights, however, result in lower paper opacity, and particularly in "strike-through" which refers to a phenomenon in which ink after printing penetrates into the back surface of paper. For the purpose of preventing these inconveniences, it has been the widespread practice to incorporate fillers during papermaking.
Such fillers are required to have a high absorption of ink, namely a high oil absorption, acid resistance to sizing, and a high whiteness, and white carbon has been widely used as a filler meeting these requirements. White carbon (synthetic amorphous silica) is roughly divided into dry, aerogel and wet method forms depending upon the method of production. But white carbons by any of these methods are relatively high in cost, and it has been desired to develop inexpensive fillers which can replace white carbon.
The present inventors have now found that a compound obtained by reacting a clay mineral of the montmorillonite group with an alkaline earth metal hydroxide and then substituting a metallic component of Groups IIb, III or IV of the periodic table for the alkaline earth metal component has a high oil absorption and a high whiteness despite its X-ray diffraction pattern (layer-like crystal structure) inherent to montmorillonite, and is useful as a filler for various purposes, particularly for lightweight paper.